From e7bce46439e3041df1bc9607e69aa9535751de92 Mon Sep 17 00:00:00 2001
From: Peter Weldon <peter.weldon@proton.me>
Date: Sun, 27 Oct 2024 17:00:04 +0000
Subject: [PATCH] Use copy_file_range if available to allow COW filesystem
 optimization.

Use case is for a near instantaneous CREATE DATABASE .. WITH TEMPLATE .. STRATEGY file_copy
---
 src/backend/storage/file/copydir.c | 70 ++++++++++++++++++++++++++++++
 src/include/storage/copydir.h      |  1 +
 2 files changed, 71 insertions(+)

diff --git a/src/backend/storage/file/copydir.c b/src/backend/storage/file/copydir.c
index d4fbe542077f1..5c58b123b871b 100644
--- a/src/backend/storage/file/copydir.c
+++ b/src/backend/storage/file/copydir.c
@@ -19,13 +19,19 @@
 #include "postgres.h"
 
 #include <fcntl.h>
+#include <sys/stat.h>
 #include <unistd.h>
 
 #include "common/file_utils.h"
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "storage/copydir.h"
+
+#include <limits.h>
+#include <common/logging.h>
+
 #include "storage/fd.h"
+#include "pg_config.h"
 
 /*
  * copydir: copy a directory
@@ -71,7 +77,11 @@ copydir(const char *fromdir, const char *todir, bool recurse)
 				copydir(fromfile, tofile, true);
 		}
 		else if (xlde_type == PGFILETYPE_REG)
+#if defined(HAVE_COPY_FILE_RANGE)
+			copy_file_reflink(fromfile, tofile);
+#else
 			copy_file(fromfile, tofile);
+#endif
 	}
 	FreeDir(xldir);
 
@@ -214,3 +224,63 @@ copy_file(const char *fromfile, const char *tofile)
 
 	pfree(buffer);
 }
+
+/*
+ * copy one file using copy_file_range to give filesystem a chance to do COW optimization
+ */
+void
+copy_file_reflink(const char *fromfile, const char *tofile)
+{
+#if  defined(HAVE_COPY_FILE_RANGE)
+	int			srcfd;
+	int			dstfd;
+	ssize_t		nbytes;
+
+	/*
+	 * Open the files
+	 */
+	srcfd = OpenTransientFile(fromfile, O_RDONLY | PG_BINARY);
+
+	if (srcfd < 0)
+		ereport(ERROR,
+				errcode_for_file_access(),
+				errmsg("could not open file \"%s\": %m", fromfile));
+
+	dstfd = OpenTransientFile(tofile, O_RDWR | O_CREAT | O_EXCL | PG_BINARY);
+
+	if (dstfd < 0)
+		ereport(ERROR,
+				errcode_for_file_access(),
+				errmsg("could not create file \"%s\": %m", tofile));
+
+	/*
+	 * Do the data copying.
+	 */
+	do
+	{
+		/* If we got a cancel signal during the copy of the file, quit */
+		CHECK_FOR_INTERRUPTS();
+
+		pgstat_report_wait_start(WAIT_EVENT_COPY_FILE_READ);
+		nbytes = copy_file_range(srcfd, NULL, dstfd, NULL, SSIZE_MAX, 0);
+		pgstat_report_wait_end();
+		if (nbytes < 0)
+			ereport(ERROR,
+					errcode_for_file_access(),
+					errmsg("could not copy file \"%s\": %m", fromfile));
+	}
+	while (nbytes > 0);
+
+	if (CloseTransientFile(dstfd) != 0)
+		ereport(ERROR,
+				errcode_for_file_access(),
+				errmsg("could not close file \"%s\": %m", tofile));
+
+	if (CloseTransientFile(srcfd) != 0)
+		ereport(ERROR,
+				errcode_for_file_access(),
+				errmsg("could not close file \"%s\": %m", fromfile));
+#else
+	pg_fatal("copy_file_range not available on this platform");
+#endif
+}
diff --git a/src/include/storage/copydir.h b/src/include/storage/copydir.h
index a25e258f4797e..fb831f363b05e 100644
--- a/src/include/storage/copydir.h
+++ b/src/include/storage/copydir.h
@@ -15,5 +15,6 @@
 
 extern void copydir(const char *fromdir, const char *todir, bool recurse);
 extern void copy_file(const char *fromfile, const char *tofile);
+extern void copy_file_reflink(const char *fromfile, const char *tofile);
 
 #endif							/* COPYDIR_H */